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4. System Design
4-2-3. Selection criteria for heat exchanger
Step 1 Determination of prerequisites for selection
1. Heat exchanger capacity 40000 W (136000 BTU/h)
2. Estimation of outlet hot water and inlet water temperatures
As a guide, select a heat exchanger of which the temperature difference between the high temperature section
and the low temperature section will be 5°C (9°F) or below.
2-1. Outlet hot water temperature (when secondary side outlet hot water temperature is set to 65°C (149°F) (setting
at the time of shipment))
Secondary side circuit outlet hot water temperature: 65°C (149°F)
Unit outlet hot water temperature: 70°C (158°F)
2-2. Inlet water temperature
Secondary side inlet water temperature: 10°C (50°F)
Unit inlet water temperature: 15°C (59°F)
3. Used flow rate
(40000 W/(70-15)°C/4200 J/kg•K) × 60 s = 10.4 kg/min ≈ 10.4 ℓ/min ≈ 2.74 GPM
Step 2 Determination of model
Notes on selection
Select a heat exchanger that allows water to pass through both of the flow channels.
Select a heat exchanger so that the pressure applied to the heat exchanger in the on-site system will not
exceed the maximum operating pressure of the heat exchanger.
Select a heat exchanger that allows flowing at a flow rate of maximum 30 ℓ/min (7.9 GPM).
Select a heat exchanger with a capacity of at least 40000 W (136000 BTU/h).
Ensure that the shearing stress at the flow rate to be used will be 16 Pa (0.01 ftAq) or more. (Refer to step 4.)
* To increase the shearing stress:
When the area per plate is equal, select a vertically long heat exchanger.
Select a heat exchanger of which NTU is high (although the heat transfer capacity improves as NTU
increases, the pressure loss becomes high).
Step 3 Determination of specifications of the heat exchanger
Determine the model of heat exchanger and number of plates in consultation with the heat exchanger
manufacturer based on the above requirements.
* To determine the number of plates, calculate the number of plates while referring to the example below.
Values to use when determining the number of plates:
1) Overall heat transfer coefficient of corresponding heat exchanger
2) Heat transfer area per plate
Calculation method
a) Obtain the data of 1) and 2) from the heat exchanger manufacturer.
b) Estimate the number of plates of the heat exchanger.
c) Check that the number of transfer units for the corresponding number of plates matches between NTU1 and
NTU2 (NTU1=NTU2).
If they are matched, select a heat exchanger having the corresponding number of plates. If they are not
matched, change the number of plates and then return to B to perform the calculation again.
MEES22K030
∆T1: Temperature difference between inlet and
outlet
∆T: Temperature difference of high temperature
part (low temperature part)
K: Overall heat transfer coefficient (W/m
A: Total heat transfer area (m
V: Total mass flow rate (kg/s) (lb/s)
C: Specific heat (J/kg•K) (BTU/lb•°F)
2
2
K) (BTU/ft
•°F•n)
2
) (ft
2
)
39
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